1. Field of the Invention
The invention relates to the field of electrical controls and enclosures having means for maintaining electrical components within an operating range of temperature. More particularly, the invention concerns elevator electrical controls thermally coupled to a circulating heat transfer system for permitting operation of the elevator for a period of time during a fire in a building.
2. Prior Art:
All modern high rise buildings have elevators for transporting people and equipment between the various floors in the building. Even buildings having only a few floors may have elevators to permit easier access to upper floors, especially for handicapped and elderly people. Federal legislation regarding building access mandate the use of elevators. However, it is unsafe to use conventional elevators to evacuate multiple story buildings during fires and other emergencies when speedy evacuation is most critical.
There are several aspects of fires that can render elevator transport unsafe, including problems with rapid smoke development and circulation, heat and structural damage, any of which can trap elevator occupants under life threatening conditions. Unless precautions are taken as to these aspects, the elevators may cease to function during fires and other emergencies, and occupants of the building, especially those trapped in the elevators, can suffer injury or death.
It is known to construct buildings having separate ventilation systems coupled to the elevator shafts and elevator lobbies, the shafts and lobbies being constructed of fire resistant materials and being pressurized slightly above ambient pressure to prevent inflow of smoke. In this manner people entering or emerging from an elevator during a building fire are not confronted by smoke and flame from the fire. Buildings can be provided with emergency backup electrical generators to provide power for operating the elevators during an emergency.
All elevator systems have electric drive motors and control systems coupled between the motors and the elevator cars, for raising and lowering the cars and opening and closing the car doors. Typically a central control device such as a microcomputer senses switch inputs from the cars and from call buttons in the elevator lobbies and the like, and provides control outputs to motor starter relays or other high current switches (e.g., SCRs, triacs or the like). The central control device is typically located in an elevator machine room above the elevator shafts where the electric drive motors are located.
Modern elevator controls including the central control and motor starter switches generally include solid state components, the operation of which is in general temperature dependent. Semiconductor devices such as integrated circuits, transistors, SCRs, triacs and the like are susceptible to malfunction due to heat. Thermal runaway conduction can occur if such devices are operated above their rated temperatures, potentially causing permanent damage due to overheating or damage to the controlled equipment due to generation of inappropriate control outputs. Under elevated temperature conditions (for example over 104.degree. Fahrenheit) the critical logic and timing functions of a microprocessor are subject to malfunction. Temperatures in the range of possible problems are easily generated by fire conditions, and also can be generated by simple weather conditions, for example if the HVAC system in the building breaks down on a hot day. The prior art methods of ensuring continued elevator operation for evacuation or fire fighter access during a fire do not provide for maintaining critical elevator controls at a safe operating temperature. Whereas elevator machine rooms where the controls are normally located are particularly susceptible to fires (e.g., due to the concentration of equipment and materials there), the most extensive use of ventilation and elevator shaft sealing steps may fail as a practical matter due to a heat-induced failure of the solid state controls which sense switch inputs and operate the elevator motors and doors. Thus, there is a need for providing an elevator control apparatus which maintains critical solid state components at a safe operating temperature even during a fire in the immediate area, at least for a period of time sufficient to evacuate all people from the building before other occurrences such as shorting of burned wires or falling debris render even properly controlled elevators inoperative.
Systems for cooling electrical devices and control cabinets are known per se. The cabinets housing temperature sensitive devices frequently include forced air circulation means and the components which dissipate heat or are sensitive to heat can be provided with heat sinks in thermal engagement with the components. The heat sinks provide added surface area for air contact or may be thermally coupled to a common heat exchange body and cooled by a fan blowing ambient air over the heat sinks, common surface and/or electrical components. Such fan cooling cannot possibly maintain a temperature lower than ambient air temperature. During a fire, the ambient air temperature rises quickly above the operating temperature range of the electrical components. Additional or alternate means for protecting the solid state devices during a fire are needed.
Liquid immersion cooling of electrical components is known in connection with aircraft and missile control applications. Immersion cooling is complex and expensive because the electrical components must be sealed against the liquid. A simple and inexpensive method of cooling elevator control panels is needed, which takes appropriate advantage of the aspects of a building and its elevator operating system.
It is also known to include refrigeration equipment in electrical control cabinets, particularly large capacity computers. An electric heat pump air conditioner cools air blown into the cabinet or air recirculated within the cabinet. Refrigeration equipment is relatively large, expensive, and subject to maintenance requirements, which require too large an investment of money and time to be justified for cooling elevator controls against the relatively remote possibility of a fire. Refrigeration equipment releases heat into the area outside the cabinet. It is susceptible to its own malfunction due to external heat, and generally is not suitable as a safety cooling means for the control apparatus of an elevator system, for reasons of both cost and effectiveness.
A device is needed which maintains temperature conditions for a control device for an elevator, is suitable for use in an elevator machine room, and provides sufficient cooling during a fire to permit the elevator to operate at least for a period of time sufficient to evacuate occupants of the building.